(1) Field of the Invention
The present invention relates to an image forming apparatus, in particular, relates to an image forming apparatus such as an electrostatic copier, laser printer, facsimile machine or the like that includes a developing device using a dual-component developer containing a toner and a magnetic carrier and forms images using toner based on electrophotography.
(2) Description of the Related Art
Conventionally, image forming apparatuses based on electrophotography such as copiers, printers, facsimile machines and the like have been known. The image forming apparatus using electrophotography is constructed so as to form an image by forming an electrostatic latent image on the surface of a photoreceptor, e.g., photoreceptor drum, supplying toner to the photoreceptor drum from a developing device to develop the electrostatic latent image, transferring the toner image formed on photoreceptor drum by development to a sheet of paper etc., and fixing the toner image onto the sheet by means of a fixing device.
Recently, in the image forming apparatuses supporting full-color and/or high-quality images, a dual-component developer (which will be referred to hereinbelow simply as “developer”), which presents excellent charge performance stability, is often used.
This developer consists of a toner and a carrier, which are agitated in the developing device and frictionally rubbed with each other to thereby produce appropriately electrified toner.
The electrified toner in the developing device is supplied to a dual-component developer supporting member, e.g., the surface of a developing roller. The toner thus supplied to the developing roller is moved by electrostatic attraction to the electrostatic latent image formed on the photoreceptor drum. Hereby, a toner image based on the electrostatic latent image is formed on the photoreceptor drum.
Further, recently, image forming apparatuses are demanded to be made compact and operate at high speeds, hence it has become necessary to electrify the developer quickly and sufficiently and also convey the developer quickly and smoothly.
For this purpose, in order to disperse supplied toner promptly into the developer and provide the toner with an appropriate amount of charge, a circulating type developing device has been adopted in some image forming apparatuses.
This circulating type developing device includes: a developer conveying passage in which the developer is circulatively conveyed; a screw auger (developer conveying member) for conveying the developer while agitating the developer in the developer conveying passage; a toner supply port for leading toner from a toner container into the developer conveying passage; and a toner concentration detecting sensor for detecting the toner concentration in the developer. In this arrangement, when the toner concentration in the developer is lower than a predetermined level, a toner supply command is given to the toner cartridge so that toner is supplied to the developer conveying passage and the supplied toner is conveyed whilst being agitated (see Patent Document 1).
There is also another disclosure of an image forming apparatus including: a mechanism in which “a comparison is made between the output value from a toner concentration sensor before the supplied toner reaches the toner concentration sensor and the output value from the toner concentration sensor after the supplied toner has reached the toner concentration sensor, and it is determined that no toner remains when the output difference becomes lower than a predetermined value”; and another mechanism in which “the toner supply time and the number of times of toner supply is varied depending on the number of times no toner is determined to remain”, and being constructed such that the interval of modifying the image forming conditions for preventing reduction in image density is shortened by comparing the reduction in toner concentration to a previously predicted and predetermined value to thereby maintain image density continually (see Patent Document 2).
Patent Document 1:
    Japanese Patent Application Laid-open 2006-106194Patent Document 2:    Japanese Patent Application Laid-open Hei 9 No. 269646
In the aforementioned circulating type developing device using the dual-component developer, if toner to be supplied from the toner cartridge to the developing device is used up, the toner concentration in the developer gradually decreases. Since the occurrence of carrier phenomena (carrier adherence) to the photoreceptor drum increases with the decrease of toner concentration, it is necessary to perform toner empty detection.
Toner empty detection is to determine (detect) the occurrence of a toner empty state when, for example, the toner concentration of the developer in the developing device, detected by the toner supply detecting sensor does not increase even after a toner supply command was given to the toner cartridge.
However, in the case where no toner is supplied even after a toner supply command was given to the toner cartridge because of toner empty in the toner cartridge, if the toner concentration detecting sensor is located away from the toner supply port through which toner is supplied, detection of toner empty is delayed because the fall of toner concentration detected by the toner concentration detecting sensor is sluggish. As a result, there occurs the problem that the occurrence of carrier adherence becomes more frequent.
Further, there is also a problem that if the amount of toner to be supplied is low, the density of the toner image formed on the paper etc. becomes lowered even before toner empty is detected.